The present invention relates to the reception of a weak radio frequency (RF) signal in the presence of a strong internal RF interferer, and in particular to the cancellation of internal interfering signals by a superposition method.
Currently, there exists an increasing need for the users of mobile phones to receive information about the exact geographical position. Therefore, it has been proposed to add a positioning system receiver unit to a mobile phone unit to localize the position of the mobile phone.
Here, the positioning system receiver is adapted, e.g., to the Global Positioning System (GPS) standard, in operation during the entire operation time of the mobile phone to ensure high accuracy and fast update of the positioning procedure. Further, the mobile phone unit itself consists of a receiver part and a transmitter part both being connected to an antenna. In case a call is established, the mobile phone unit starts to transmit a RF signal at a dedicated frequency and at well defined time slots, e.g., according to the TDMA/FDMA transmission scheme defined for the GSM standard.
During transmission the transmitter in the mobile phone unit uses at least a single time slot per frame but it is also planned to use a plurality of time slots to send data in the future. This means, in an extreme case the mobile phone is transmitting continuously.
While up to now the positioning system receiver unit and the mobile phone unit are implemented into physically separated devices there is the tendency to bring the positioning system receiver unit and the mobile phone unit together.
However, if the positioning system receiver unit and the mobile phone unit are integrated in a multiple standard communication device, e.g., through attaching the positioning system receiver unit to the mobile phone unit with a dedicated connector or even by building the positioning system receiver unit into the housing of the mobile phone unit additional problems arise due to the decreased distance between both functional units.
FIG. 1 shows a schematic diagram for a multiple standard communication device 100 having multiple functional units using different standards. Typically, the multiple standard communication device comprises at least two sub-units 102 and 104 each having an antenna 106 and 108, respectively. As outlined above, for such a multiple standard communication device, e.g., one sub-unit is a dedicated positioning system receiver unit in the group of GPS, Glonass, EGNOS, WAAS, etc., and the other sub-unit is a mobile communication system unit in the group of GSM900, GSM1800, GSM1900, AMPS, DAMPS, PDC, CDMA, etc.
Here, the transmission signal generated by the mobile communication unit 104 disturbs the functionality of the positioning system receiver unit 102 in the communication device 100. The reason herefor is that an antenna 106 of the mobile phone 104 and an antenna 108 of the positioning system receiver 102 are located close to each other due to the small dimensions of the communication device 100. Therefore, the isolation between both antennas 106 and 108 is limited to a certain value, e.g., lying in the range of 30 to 40 dB.
Since the output power of the mobile phone 104 may reach 33 dBm or even more, this means that a very strong transmission signal is transmitted by the mobile communication unit via the related antenna 106 and reaches the neighbouring antenna 108 of the positioning system receiver unit 102. This leads to a relatively strong and undesired interfering signal that changes the reception behaviour in the positioning system receiver 102.
As shown in FIG. 2, the first elements of the receiver part of the positioning system receiver unit 102 consists of an antenna 108, a low noise amplifier 110 and a subsequent frequency conversion unit 112. In case a strong interfering signal is received via the antenna 108 of the positioning system receiver unit 102, this interfering signal changes the operation point of the low noise amplifier 110 in the positioning system receiver 102 and thus deteriorates the gain and noise figure for the desired incoming positioning system localization signal which itself is already very weak, i.e. below xe2x88x92120 dBm.
Another source for the occurence of interferences may be the radiation of interfering signals from any part of the transmitter. These interference signals may be received via any line segment of the positioning system receiver.
The positioning system receiver thus gets xe2x80x9cblindxe2x80x9d for the weak positioning system localization signal orxe2x80x94in other wordsxe2x80x94the sensitivity of the positioning system receiver for the localization signal is decreased significantly during transmission time periods of the mobile phone 104. This in conclusion means that the positioning functionality of the positioning system receiver 102 is severely affected.
In view of the above the object of the present invention is to compensate for any interference from a mobile communication system unit to a positioning system receiver unit in a multiple standard communication device with mobile communication and localization functionality.
Therefore, according to the present invention, it is proposed to branch off, e.g., with any type of coupling device, a part of the transmitted output signal in a second sub-unit before the antenna. The branched-off is attenuated/amplified and phase-shifted in such a way that it has a magnitude and sign/phase to compensate for interference caused by unwanted signal at the input of a superposition unit in the receiver part of a first sub-unit for receiving weak signals. Therefore, interfering signal compensation is achieved through superposition of the interfering signal with the attenuated amplified and phase-shifted branched-off output signal.
According to a preferred embodiment of the present invention the input signal is the input signal of a positioning system receiver unit for localization purposes and the output signal is a transmission signal of a mobile phone unit. Preferably, the input signal may be of the global positioing type and the transmission signal may be of the GSM type. Also, the present invention is as well suited for application to a dual mobile communication standard, e.g, GSM 900/GSM 1800 or GSM 900/GSM 1900.
Since in the superposition unit both the desired positioning system localization signal superposed with an interferer signal from the mobile communication system and the phase-shifted and attenuated branched-off transmitter signal are superposed, the disturbing component in the reception signal of the positioning system receiver unit ideally may be cancelled out in full so that the positioning system receiver unit remains fully operational also during transmission operation of the mobile communication unit. Therefore, the localization functionality of the mobile device is ensured also during time periods of transmission for the mobile communication unit.
According to a preferred embodiment of the present invention, the attenuation/amplification and phase-shifting is also carried out to compensate for time-variant external influences through minimizing the signal at the output of the superposition unit when the phase difference is modified in a range between 0 and 2xcfx80 in the phase-shifter and the magnitude is attenuated and amplified.
Therefore, this approach achieves a minimum disturbance level at the output of the superposition unit irrespective of whether the interfering signal is initiated through the mobile communication unit or received from external sources operating at the same transmission frequency as the internal communication unit via the antenna of the positioning system receiver unit. Also, in case of multiple disturbance sources, the minimum disturbance level in the positioning system receiver is maintained.
The signal at the output of the superposition unit is used as a basis for interference compensation. Therefore, the parasitic inductance and capacitance of line segments between, e.g., the power amplifier of the mobile communication unit to the branched-off unit and further line segments between the positioning system antenna and the superposition unit as well as the parasitic circuit elements according to the connections to the controllable attenuator and phase-shifter are taken into account so that no disturbances are introduced at the positioning system receiver unit through the error compensation itself.
The iterative shifting of the phase between 0 and 2xcfx80 and of the amplitude is implemented by software code portions running on a processor unit for performing the necessary iterative steps. These software code portions are integrated into a computer program product directly loadable into the internal memory of the processor unit when the computer program product is run on the processor unit.
Preferably the computer program product may be stored on a computer usable storage medium comprising the computer-readable software code portions for performing the inventive subtraction method when the computer program product is run on the processor unit.
Therefore, using this flexible implementation of the present invention it is possible to easily modify iterative strategies to achieve the minimum disturbing signal at the output of the superposing unit without any modification of the hardware structure underlying the receiver apparatus according to the present invention.
Also, the inventive superposition method stored on the computer-usable storage medium may be easily transferred to a plurality of receiver apparatus, e.g., after a modification to achieve a update of the receiver apparatus functionality again without any receiver apparatus hardware modifications.